Power semiconductor devices, such as power MOS (metal oxide semiconductor) transistors or power diodes, include a drift region and a pn junction between the drift region and a body region in an MOS transistor and between the drift region and an emitter region in a diode. The doping concentration of the drift region is lower than the doping concentration of the body and emitter region, so that a depletion region (space charge region) mainly expands in the drift region when the device blocks, which is when the pn junction is reverse biased.
The dimension of the drift region in a current flow direction of the device and the doping concentration of the drift region mainly define the voltage blocking capability of the semiconductor device. In a unipolar device, such as a power MOSFET (metal oxide semiconductor field effect transistor), the doping concentration of the drift region also defines the on-resistance of the device, which is the electrical resistance of the semiconductor device in the on-state.
When the pn junction is reverse biased dopant atoms are ionized on both sides of the pn junction resulting in a space charge region that is associated with an electrical field. The integral of the magnitude of the field strength of the electrical field corresponds to the voltage that reverse biases the pn junction, where the maximum of the electrical field is at the pn junction. An Avalanche breakthrough occurs when the maximum of the electrical field reaches a critical field strength that is dependent on the type of semiconductor material used to implement the drift region.
The doping concentration of the drift region may be increased without reducing the voltage blocking capability of the device when charges are provided in the drift region that may act as counter charges to ionized dopant atoms in the drift region when the pn junction is reverse biased so that a depletion region expands in the drift region.
According to a known concept, field electrodes or field plates are provided in the drift region and are dielectrically insulated from the drift region by a field electrode dielectric. These field electrodes may provide the required counter charges.
According to one known concept, these field electrodes are electrically connected to a fixed electrical potential, such as gate or source potential in a MOSFET. However, this may result in a high voltage between the field electrode and those regions of the drift region close to the drain region in a MOSFET, so that a thick field electrode dielectric would be required. A thick field electrode dielectric, however, reduces the desired compensation effect.
According to a further known concept, several field electrodes are arranged distant to each other in a current flow direction of the drift region and these field electrodes are connected to different voltage sources, so as to bias these field electrodes to different potentials. Implementing the voltage sources, however, is difficult.
According to yet another known concept, the field electrodes are electrically connected to a doped semiconductor region of the same doping type as the drift region through a contact electrode arranged above a semiconductor body. This “coupling region” is at least partially shielded against the drift region by a semiconductor region of a complementary doping type.
There is a need to reduce the on-resistance and to increase the voltage blocking capability of a semiconductor device with a drift region.